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Directed Crystallization of CuSO4 3 5H2O onto Carbon Nanotube Microarchitectures Sharon Xiaodai Lim,†,# Yong Hui Lim,‡ Chang Le Charlotte Loh,‡ Ying Yi Wendy Yap,‡ Guo Hao Chia,#,‡ and Chorng-Haur Sow*,†,# †

NUS Graduate School for Integrative Sciences & Engineering (NGS), Centre for Life Sciences (CeLS), #05-01, 28 Medical Drive, Singapore 117456 # Department of Physics, Blk S12, Faculty of Science, National University of Singapore, 2 Science Drive 3, Singapore 117542 ‡ Dunman High School, 10 Tanjong Rhu Road, Singapore 436895

bS Supporting Information ABSTRACT: An assembly technique based on crystallization of thin CuSO4 3 5H2O microcrystals from salt solution onto patterned three-dimensional (3D) multiwalled carbon nanotube (MWNT) platform was developed. The vertically aligned MWNT arrays served as nucleation sites for the formation of microstructured CuSO4 3 5H2O crystals. In the presence of MWNTs, the CuSO4 3 5H2O crystal exhibited preferential crystallization in the (222) orientation. Sculpting the MWNT platforms using a focused laser beam, as well as utilizing capillary forces that occurred during the drying process of the salt solution, a variety of 3D MWNT-microcrystal hybrid systems were created. In addition to the 3D MWNT platform created by focused laser beam pruning, we have also achieved assembly of the microcrystals onto MWNT array with patterned hydrophilic and hydrophobic regions. Such chemical modifications of the MWNT surface were achieved without introducing any physical destruction to the MWNT arrays. As such, two-dimensional template-directed assembly of crystals onto the hydrophilic regions on the MWNT surface was realized.

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elf-assembly, a tool used to build micro- and nanomaterials into ordered macroscopic structures, has been frequently utilized in the process of structural material fabrications. Material fabrications are progressing into a phase where well-defined microstructures created using nanoscale materials are becoming a key technology.1 The driving forces behind such self-assembling phenomena can be attributed to the need for materials to achieve a state of lower free energy and greater structural stability. To add a degree of versatility to such an assembly process, patterned templates have often been introduced to serve as a means to direct the formation of a specific structure, to balance charge, or even to fill space.2 An example of such template directed assembly could involve the use of patterned planar substrates, where spatially defined hydrophobic hydrophilic interactions between substrate and building blocks guided the assemblage onto predetermined architectures.3,4 In the process of fabricating advanced organic and inorganic materials, control over the crystallization process is an important criterion that has to be met. Through a combination of soft lithography and self-assembled monolayers (SAMs) of different termination, crystals can be engineered to nucleate with specific density, size, orientation, and designed pattern. Such patterning is crucial for implementation in electronic, sensory, and optical devices.5 In a report by Aizenberg et al.,6 calcite crystals were directed to nucleate at acid-terminated regions through microcontact printing via an elastometric stamp. Varying the area and distribution of the acid-terminated regions, control over the density of the crystals r 2011 American Chemical Society

can be achieved. In addition, the authors were able to manipulate the crystallographic orientation through the use of different functional groups and substrates. An alternative method of achieving patterned continuous growth of calcium carbonate thin films has also been reported by Lee et al.7 In their work, the shape and polymorph of the calcium carbonate films synthesized on sitespecific patterned SAMs on gold substrates were predetermined by acid-terminated gold nanoparticles. These nanoparticles were bounded to the SAMs through carboxylate (on nanoparticles) Ca2+ - carboxylate (on SAMs) linkages. While these methods provided avenues for the creation of crystals on patterned substrates, the patterns created were mainly two-dimensional. To provide flexibility in the implementation of these materials for device fabrications, the ability to synthesize these organic/ inorganic materials into three-dimensional (3D) structures with controlled hierarchy is desired. In a publication by Sakamoto et al.,8 patterned 3D relief structures of CaCO3 were formed by spontaneous two-step crystal growth on a polymer matrix in the presence of an acidic polymer. In the initial phase, a thin film of crystals displaying periodically changing crystallographic orientations was prepared on a flat substrate. Following which, relief structures comprising of needle-like crystals are spontaneously assembled onto the thin film of crystal. Received: July 3, 2011 Revised: September 5, 2011 Published: September 20, 2011 20964

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Figure 1. (a) Schematic of the experimental setup for the assembly process. (b) SEM and (c) EDX map (element, sulfur) of the cross-sectional view of the MWNT platform showing that the layer of CuSO4 3 5H2O crystals was preferentially assembled onto the top surface of the platform.

Carbon nanotubes (CNTs) with their unique mechanical, electrical, and optical properties,9 13 have captivated many researchers ever since their discovery.14 With CNTs exhibiting a wide range of potential applications including gas storage,15 particle sieving,16 and electro-mechanical sensing,17 a hybrid 3D system created using CNTs and inorganic crystals further extend the range of potential hybrid nanomaterial system for more potential applications. CNTs and nanocrystals hybrid systems have been reported to show potential applications in biosensors,18 chemical sensors,19 and photovoltaic devices.20 As such, CNTs have often been used as a substrate to grow nanocrystals.21,22 Some of the fabrication methods include heat treatment,21 thermal evaporation deposition,22 and chemical vapor transport.23 However, these methods often involved the use of heating at high temperature and in a controlled gaseous environment. With many microelectronic systems becoming 3D, the development of assembling techniques that can achieve controlled 3D hierarchical structure of CNT/crystals hybrid would be very useful. In this work, an assembly technique that involves recrystallization of a thin, microstructured uniform film of CuSO4 3 5H2O crystals (previously dissolved in distilled water) onto micropatterned functionalized CNT arrays was developed. CuSO4 3 5H2O crystals were selected for this work because CuSO4 3 5H2O has been known to serve as an intermediary in the electronics industry.24 In this process, the 3D vertically aligned CNT arrays served as nucleation sites for the formation of microstructured CuSO4 3 5H2O crystals. As the process is being carried out at room temperature and in ambient conditions, this method of directed recrystallization thus provided greater versatility for implementation in device fabrication processes. In addition, presence of the CNTs resulted in preferential orientation of the crystals as they recrystallized on the CNT microplatforms. Using a flexible laser pruning technique, the shape, size, and configuration of the CNT microstructures can be controlled. With successful assembly of the CuSO4 3 5H2O crystals onto the CNT microstructures, the eventual CNT/crystal hybrid architecture can thus be predetermined. To allow the salt solution to spread onto the hydrophobic CNT surface, the CNT samples were pretreated with oxygen plasma. This changes the CNT surface from being hydrophobic to being hydrophilic. By using a laser power of 5 mW to raster selected regions on the oxygen functionalized CNT surface, the hydrophobic nature of the CNTs can be recovered at these specific sites without causing any destruction to the CNT arrays. In doing so, template directed assembly, where CuSO4 3 5H2O crystals were guided onto predetermined hydrophilic architectures, can be achieved.

’ RESULTS AND DISCUSSION Directed Assembly of Uniform CuSO4 3 5H2O Crystals onto MWNT Platform. Technique for Directed Crystal Assembly. The

surface of MWNTs is hydrophobic in nature. By treating the sample with oxygen reactive ion etching (O2RIE) for 30 s, the surface of the MWNTs became hydrophilic. From our previous work,25,26 it was determined that some hydrophilic chemical bonds such as OH CdO were formed at the open-ends of MWNTs, indicating an important factor contributing to the fast wettability change after the O2 RIE treatment. As a result, the water-based CuSO4 3 5H2O solution would be directed to nucleate across the hydrophilic MWNT surface. In addition to the surface state assisted assembly of the MWNT crystals, factors contributing to the success of this assembly process could also include electrostatic interactions between the surface OH CdO functional groups and the CuSO4 3 5H2O crystals. To assemble CuSO4 3 5H2O crystals onto the patterned MWNT with microplatforms, an inverted MWNT sample was attached to a holder that allows gradual lowering of the sample onto a thin film of the CuSO4 3 5H2O solution formed around the center of a small metal loop (Figure 1a). Dipping the small metal loop into the CuSO4 3 5H2O solution and lifting it out, surface tension allowed the formation of the thin film of CuSO4 3 5H2O solution on the metal loop. The purpose of creating the film on the metal loop was to provide control over the amount of solvent in contact with the surface of the MWNTs, as well as to prevent the formation of water meniscus over the MWNT surface. Once the contact with the thin film of solvent was established, the MWNT sample was removed and left to dry in an inverted manner. The entire assembly and drying process were carried out in ambient conditions. The result was the successful formation of a uniform layer of CuSO4 3 5H2O crystals onto the MWNT surface as seen from the cross-sectional view of the MWNT platform (Figure 1b). Figure 1c shows the energy dispersive X-ray (EDX) map of the element, sulfur; from Figure 1b, the thickness of the crystal layer was determined to be 4.4 μm. Detailed studies of alternative drying methods for the formation of a uniform film of crystals on the MWNT platforms can be found in the Supporting Information. Optimum Length and Width for Crystal Assembly. To investigate the presence of any limitation for optimum crystal coverage, the laser pruning technique was implemented to cut the MWNT array into rectangular platforms, with length ranging from 100 to 400 μm and width varying from 10 to 80 μm. After the assembly process, SEM images of MWNT platforms with similar width (20 μm) but length of 100 μm (Figure 2a) and 20965

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The Journal of Physical Chemistry C 400 μm (Figure 2b) show that uniform coating of microstructured crystals were successfully assembled onto both platforms. The result thus indicated that uniform distribution of CuSO4 3 5H2O crystals on MWNT platforms were independent of the length of the platforms. Instead, the uniformity of the crystals was dependent on the width of the MWNT platform on which it is being assembled. From Figure 2c, MWNT platforms with width ranging from 10 to 80 μm (indicated by the dotted arrows) and a fixed length of 400 μm were prepared. Upon the assembly of crystals, capillary induced densification of MWNTs26 was observed for all platforms. Such densification effect resulted in the reduction of the overall dimensions of the crystal layer on the platforms. In the case of 10 μm MWNT platform, the effect of such densification was so intense that no platform was available for the salt solution to recrystallize. The Effect of Concentration of CuSO4 Solution. Different concentrations of CuSO4 3 5H2O solution (0.05, 0.15, 0.25, 0.35,

Figure 2. (a c) SEM images showing uniform crystal assembly on MWNT platforms of starting width of 20 μm and length (a) 100 μm and (b) 400 μm. (c) Assembly of crystal layers were also observed on MWNT platforms with length of 400 μm and widths ranging from 20 to 80 μm (as indicated by the solid arrows). No crystals were assembled onto the 400  10 μm MWNT platforms as a result of capillary induced distortion of the MWNT platform. Dotted arrows indicate the original width of the MWNT platforms.

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and 0.45 g cm 3) were used to investigate the optimum concentration required for the formation of a uniform film of CuSO4 3 5H2O microstructured crystal on the MWNT platforms. The concentration test was conducted on MWNT platforms of similar width. Using a concentration of 0.05 g cm 3, a minimal amount of crystals was deposited onto the MWNT platform (Figure 3a). By increasing the concentration of the crystals to 0.15 g cm 3, thin and uniform coverage of the MWNT platform was achieved (Figure 3b). When the concentration of the crystals was increased to 0.25 g cm 3 (Figure 3c) and 0.35 g cm 3 (Figure 3d) uneven formation of crystal layer on the MWNT platform was observed throughout the entire MWNT platform. This could be attributed to the presence of more nucleation sites for the crystallization process to take place as the concentration of the crystal solution increases. Increasing the concentration to 0.45 g cm 3 (Figure 3e), a thick and nonuniform crystal film was formed on the middle section of the MWNT platform. The double headed arrow indicates the width of this thick crystal film. As such, it could thus be concluded that a thin and uniform crystal layer could be assembled onto MWNT platforms by employing solution with a concentration of 0.15 g cm 3. XRD and TEM Analysis of the MWNT/Crystal Hybrid Structures. For further analysis, the X-ray diffraction (XRD) patterns of the purified crystals and the MWNT/crystal hybrid were recorded (Figure 4a). Both the purified crystals as well as those assembled onto the MWNTs array were obtained from the same salt solution with the optimum concentration of 0.15 g cm 3. The purified crystals were obtained by allowing the salt solution to recrystallize in ambient conditions. From Figure 4a, crystals formed on the MWNTs array exhibit a very strong (222) peak, whereas this peak was very weak for the purified crystals. Such preferential orientation of the crystal structures in the presence of the MWNT arrays could be attributed to epitaxial matching between the specific crystal plane and the MWNTs interface. From the XRD patterns, it can be determined that the resultant CuSO4 3 5H2O crystal layer was an oriented architecture of smaller crystallites. Size of the crystallites as calculated from the XRD patterns was approximately 163 nm. By scratching MWNT/crystal hybrid structures onto the TEM grid, TEM analysis were also conducted. Figure 4b shows an

Figure 3. Distribution of recrystallized crystals on MWNT platforms with a starting solution of varied concentrations, (a) 0.05 g cm 3, (b) 0.15 g cm 3, (c) 0.25 g cm 3, (d) 0.35 g cm 3, (e) 0.45 g cm 3, of CuSO4 3 5H2O crystals dissolved in distilled water. Double headed arrow in (b e) indicates the location of the crystal layer on the MWNT platform. 20966

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Figure 4. (a) XRD patterns of recrystallized CuSO4 3 5H2O crystals with and without the presence of MWNTs. (b) TEM imaging of a CuSO4 3 5H2O crystal on MWNTs. Inset is a higher magnification of the crystal enclosed within the white box, showing lattice structures of the CuSO4 3 5H2O crystal.

image of a broken piece of CuSO4 3 5H2O crystal recrystallized onto a few strands of MWNTs. Inset shows a higher magnification image taken from the edge of the crystal. As the crystal formed on the MWNTs was too thick for the TEM imaging, no crystal lattice was observed. Controlled Assembly of CuSO4 3 5H2O Crystals on MicroPatterned MWNT Structures. The ability to create a great variety of structures using this method would be important when it comes to designing the hybrid system for device fabrication. To illustrate the versatility of such an assembly process, more complicated structures were created on the MWNT arrays via the laser pruning technique. On the basis of the above investigations, one can conclude that by utilizing the technique developed and allowing the CuSO4 3 5H2O crystals to evaporate in ambient conditions while suspending the MWNT sample upside down, a thin film of CuSO4 3 5H2O crystals can be assembled uniformly onto MWNT platforms of width >10 μm. As such, a “Lion” structure was laser-pruned on a MWNT array (Figure 5a), and uniform CuSO4 3 5H2O microstructured crystals were successfully assembled onto the pattern (Figure 5b). Further analyses were carried out on the sample using EDX spectroscopy. EDX mapping was carried out on the sample to verify that the “Lion” structure was indeed covered with CuSO4 3 5H2O crystals. Figure 5c shows EDX mapping of element sulfur (S), and the presence of this element was indicated by green dots. The elemental map thus provided strong evidence that the “Lion” structure crafted out of the MWNT array was successfully covered by CuSO4 3 5H2O crystals. In another related experiment, groups of four MWNT pillars were also created as shown in Figure 5d. Recrystallized CuSO4 3 5H2O was assembled onto these pillars using the same technique as described in this work. During the drying process, the presence of capillary force was able to deform the MWNT pillars toward the center of the cluster at the top, and they were held together by a CuSO4 3 5H2O microcrystal formed during the drying process (Figure 5e). Figure 5f shows a lower magnification of 4  4 arrays of MWNT pillars before the introduction of the salt solution. Figure 5g presents an image of 4  4 arrays of MWNT pillars-CuSO4 3 5H2O microcrystal hierarchical structure similar to that shown in Figure 5e. With laser assisted creation of different MWNT structures, the recrystallization process can be site specific. With the ability to craft any structures using the laser pruning technique, this method of assembly thus allows a great variety of MWNT/crystal hybrid systems to be created. It also presents a mean for the production of controlled 3D hybrid architectures in ambient conditions.

Figure 5. SEM images of the “Lion” structure (a) before and (b) after undergoing the assembly process. (c) EDX elemental map of the element, sulfur on the same “Lion” structure. (c) SEM images (25° tilt) of clusters of four MWNT pillars (d) before and (e) after being held together by a thin CuSO4 3 5H2O microcrystal. (f) 4  4 arrays of MWNT pillars shown in (d) (25° tilt). (g) 4  4 MWNT pillars-CuSO4 3 5H2O microcrystal hierarchical structure shown in (e).

Controlled Assembly of CuSO4 3 5H2O Crystals onto Chemically Patterned MWNT Surface. Other than tailoring the

eventual structures of the MWNT/crystal hybrid through physical means, it was discovered that an alternative way to direct the architecture could be achieved through fine-tuning of the surface chemical properties of the MWNTs array. With an oxygen treated MWNT surface displaying a hydrophilic nature, our previous work25 has shown that by selectively cutting away the top portions of the MWNT array, the surface of the MWNT array was found to revert back to its initial hydrophobic state. However, such a recovery process was achieved at a cost of reduction in the height of the MWNTs. In this work, it was found that through the use of 20967

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Figure 6. (a c) Schematic showing (a) as-grown MWNTs being (b) treated with O2RIE before undergoing (c) laser pruning to recover the hydrophobic nature of the MWNT surface. (d) Schematic of structure pruned onto oxygen treated MWNT array. (e, f) Higher magnification of the assembly of CuSO4 3 5H2O crystals onto selective hydrophilic regions on the MWNT array. Inset of (e) shows a clear intersection between hydrophobic (MWNTs) and hydrophilic (crystal) regions while that of (f) shows distinct physical destruction on the MWNT array due to pruning at 10.0 mW laser power.

low laser power, such recovery of the hydrophobic nature of the MWNT surface could be achieved without causing any physical destruction to the MWNT array. And this feasibility was further utilized to direct preferential recrystallization of the CuSO4 3 5H2O crystal. The main feature here is the surface chemical properties are different instead of a difference in physical structure. Laser Power Dependent Study. To determine the minimum laser power required to recover the hydrophobic nature without causing any physical destruction to the MWNT surface, as-grown MWNTs (Figure 6a) were first treated with O2RIE (Figure 6b). After which, parts of the oxygen treated MWNTs were exposed to focused laser beam (Figure 6c) at different powers of 0.8, 1.0, 5.0, and 10.0 mW, respectively (Figure 6d). As illustrated in Figure 6c, at the appropriate laser power, the hydrophilic MWNT could be converted to a hydrophobic surface without any physical damage to the MWNT. Subsequently, recrystallization of CuSO4 3 5H2O onto the treated surface was carried out. After the assembly of CuSO4 3 5H2O crystals (Figure 6e,f), regions pruned using 0.8 and 1.0 mW laser power were completely covered by CuSO4 3 5H2O crystals. This implies that the laser powers used were too weak and thus ineffective in reverting the hydrophilic nature of the treated MWNTs. On the other hand, siteselective assembly of crystals could be seen in regions patterned using 5.0 and 10.0 mW laser power. In particular, the crystal avoided the regions that were previously exposed to a laser beam. These regions are more hydrophobic; hence the crystals were found to assemble onto the neighboring hydrophilic regions. From the higher magnification view of the sample (Figure 6e), while no physical destruction to the MWNT surface was observed for the structure exposed to 5.0mW laser power, laser paths could be spotted on the MWNT surface when 10.0 mW laser power was used (Figure 6f). As such, laser power of 5.0 mW was adequate to revert the oxygen treated MWNT surface back to a more hydrophobic state without any physical destruction. With the flexibility provided by the laser, it opens up the possibility to use this technique as a form of visual coding. Information could be encoded on the MWNTs array while remaining optically “invisible” before the introduction of the salt solution. Upon introduction of the solution, the encoded pattern would reveal itself upon the drying of the solution. To understand how laser pruning with a laser power of 5.0 mW could change the chemical states on the surface of the MWNTs,

such that the hydrophobic nature of oxygen treated MWNT was recovered, XPS analyses were conducted on as-grown, O2RIE treated, and laser pruned O2RIE treated MWNTs. Comparing the O 1s curves obtained (Figure S2, Supporting Information), a reduction in O 1s peak of O2RIE MWNTs after undergoing 5.0 mW of laser pruning was observed with respect to O2RIE MWNTs without undergoing any laser pruning. As such, MWNT surfaces that had undergone laser pruning would exhibit a chemical state that is relatively more hydrophobic as compared to the surrounding MWNT surfaces. Thereby, providing a means of directed recrystallization of the CuSO4 3 5H2O onto the hydrophilic regions.

’ CONCLUSION In summary, techniques to assemble a thin, uniform CuSO4 3 5H2O crystal film onto various 3D multiwalled carbon nanotube (MWNT) platforms are presented. The optimum concentration was determined to be 0.15 g of CuSO4 3 5H2O crystal dissolved in 1 cm3 of distilled water. To achieve such a water based assembly process, MWNTs were pretreated with an oxygen reactive ion etching process in order to attain a hydrophilic MWNT surface. By focusing a laser beam of power at 5.0 mW, the hydrophobic nature of the MWNT surface can be recovered without causing any physical damage to the nanotubes surface. As such, a visual coding system can be established. The assembly process was only limited by the width of the MWNT microplatforms. Lastly, in the presence of MWNTs, the CuSO4 3 5H2O crystals were found to exhibit preferential orientation during the recrystallization process. ’ METHODS MWNTs and Laser Pruning. Aligned multiwalled CNTs (MWNTs) with a typical height of 30 40 μm were grown on clean N-typed silicon (2.5  2.5 mm, (100) Si) substrates containing native oxide layer. Before growth, a layer of iron film (∼17 nm) was coated on the Si substrates as catalyst using a magnetron sputtering system (model: RF Magnetron Denton Discovery 18). These MWNTs were synthesized using a plasma enhanced chemical vapor deposition (PECVD) system, and details of the growth process were reported elsewhere.27,28 A laser pruning technique29 involving a focused continuous laser beam of wavelength 660 nm with 3 mm initial beam diameter and a computer 20968

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The Journal of Physical Chemistry C controlled sample stage was used to create platforms and patterns made up of arrays of MWNT. By focusing the laser beam using an optical lens, the beam size can be reduced. Setting the laser power to 5 mW, chemical modification of the MWNT surface was achieved at specific locations. Using 50 mW focused laser beam, pillars in clusters of four, and MWNT platforms were controllably pruned out of the MWNTs array. Reactive Ion Etching. In order to change the surface of MWNTs from hydrophobic to hydrophilic, oxygen reactive-ion etching is required.25,26 As-grown MWNT arrays were placed inside the chamber of a SAMCO RIE-10N Reactive Ion Etching Unit and pumped down to a base pressure of 4  10 6 Torr. O2 gas was utilized during the treatment. MWNTs were placed on a RF-driven capacitatively coupled electrode. RF power was set at 20 W and reflected power was about 1 W. Work pressure was about 0.05 Torr in the chamber and temperature was kept at 20 °C. During the RIE process, the flow rate of oxygen was set at 34.50 sccm (standard cubic centimeter per minute) with fixed durations of 30 s. Preparation of CuSO4 3 5H2O Salt. CuSO4 3 5H2O salts were dissolved in distilled water and impurities were removed via filtration. The filtrate was heated at 100 °C to one-third its original volume and left in ambient conditions for recrystallization to take place. The crystals were then pressed dry between sheets of filter paper. These crystals were subsequently dissolved in 1 cm3 of distilled water each to produce concentrations of 0.05, 0.15, 0.25, 0.35, and 0.45 g cm 3, respectively. Further Characterizations. Further characterisations of the samples were conducted using JEOL JSM-6701F field emission scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDX) function, a “Cascade Microtech” optical microscope and X-ray photoelectron spectroscopy (XPS, Kratos DLD Ultra UHV spectrometer with a monochromatic Al Kα source that has a spot size of 1 mm). To collect the X-ray diffraction (XRD) patterns of the purified crystals and the MWNT/crystal hybrid, a Siemens D5005 X-ray powder diffractometer with Cu Ka radiation (l = 0.15406 nm) was used.

’ ASSOCIATED CONTENT

bS

Supporting Information. Results obtained from the study of the effect of drying method on the resultant crystal layer formed on the MWNT surface and the XPS peaks obtained from as-grown, O2RIE treated, as well as laser pruned O2RIE treated MWNTs are presented. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: (+65) 65162957. Fax: (+65) 67776126. E-mail: [email protected].

’ ACKNOWLEDGMENT The authors acknowledge the help of Ms. Loh Pui Yee for obtaining the XRD pattern of the respective samples.

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